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Blood Pressure: What to know, and how to measure one properly.

Blood pressure is one of the four main vital signs, along with pulse, respirations, and temperature, which are aptly named to reflect their importance in the evaluation of a patient. Blood pressure is an especially important figure, as it is directly proportional to the perfusion pressure of various organs (the pressure needed to squeeze oxygen-carrying blood to vital organs). For example, if a patient’s blood pressure is too low, blood may not be able to overcome the resistance of blood vessels (and the force of gravity), in order to reach the brain. Without blood, the brain can become dysfunctional. If the lack of blood flow to the brain remains uncorrected, it can result in tissue death (stroke), which is irreversible.

Pressure and Flow - Relating it to Electricity

When you plug a device into the wall, electrical current (read: ‘flow’ of electrons) occurs through the wire. These electrons are driven to move through the wire, because they are attracted to a positive charge at the far end of the wire. The difference in charge that drives the electrons to move through the wire is known as the voltage (read: ‘pressure’ difference). In order for the electrons to move through the wire, there must be a sufficient difference in charge to drive their movement. However, there is one more component to this equation… resistance. Resistance is the force which works against a moving electron. If you were to try to move your sneakers across a carpeted floor, you would find that resistance is the force that makes it difficult to move your foot. In order to force the current to move against high resistance, there must be a sufficiently high voltage to ‘push’ them through. This is represented mathematically by Ohm’s Law.

Ohm's Law

Current = Voltage / Resistance

Let’s compare this to blood pressure —

When blood is pushed through blood vessels, it meets a certain amount of resistance. If the resistance is high (smaller blood vessels or an obstruction to flow, for example), then a greater blood pressure is needed to force the blood though.

Ohm's Law: Translated for Blood Flow

Blood Flow = Blood Pressure / Resistance

Pressure and Flow - Real World Example

We know that a greater blood pressure is needed in order to push blood past an area of higher resistance in order to maintain adequate blood flow. But how do we obtain higher resistance in the body?

One common example involves the pressures of the skull.

The Monro-Kellie Hypothesis:

The skull is basically a big, bone bowl — it doesn’t have much ability to stretch. Within the skull are three things: blood, brain, and cerebrospinal fluid. Because the skull cannot stretch, if you increase the amount of any one of those three things, then everything else must decrease (…try adding more ice to a full cup of soda without the soda spilling out). Thus, if the brain is swollen (ie. after a head injury, or due to electrolyte disturbances in the blood) and it occupies more space, there will be less room for blood and CSF. Similarly, if there is free bleeding in the skull, there will be less room for brain, blood inside vessels, and CSF, which will be squeezed out. Thus, anything that increases the pressure inside the skull will create a squeeze on the blood vessels in the brain and increase resistance to flow.

In order to protect blood flow to the brain when this resistance is increased, the body will try to increase the blood pressure to maintain blood flow. This is represented by the following equation:

Thus, if a patient has increased intracranial pressure, the body’s response is to raise the blood pressure. We cannot easily measure intracranial pressure, but we can easily measure blood pressure. If it is elevated in the proper clinical context (ie. head injury), we may consider the possibility of increased intracranial pressure.

Principle Behind Blood Pressure Measurement

Blood pressure is measured manually using a blood pressure cuff (“Sphygmomanometer”) and a stethoscope. The technique involves determining the resting pressure in the artery (“Diastolic“: When the heart is resting between squeezes), and the peak pressure in the artery (“Systolic“: When the heart is actively squeezing).

To do this, a pressure cuff is used to squeeze the arm until the arteries in the arm are completely squeezed shut. As the cuff is relaxed, it will reach a point at which the artery ‘pulses’ open with each heartbeat. At this point, the systolic (squeezing) pressure is high enough to open the artery, but it cannot stay open with resting pressure alone. This point is measured as the systolic pressure.

As the cuff is further relaxed, the artery will reach a point at which it can remain completely open with the resting pressure in the vessel. This is the diastolic pressure.

Relating Systolic and Diastolic Pressures to the Ocean's Waves

Have you ever stood in the ocean and felt the water around your legs? If you closed your eyes, you would still know that the water was there because it exerts a resting pressure against your legs. This resting pressure is equivalent to the diastolic pressure — it is the pressure of blood that is simply pooling in the arteries.

While you are standing in the ocean with your eyes closed, you would also be aware of waves lapping against your legs. You recognize the presence of waves, because they exert a higher pressure on your legs each time they hit you. This wave-like pressure is equivalent to the systolic pressure — it is the pressure of blood that is pushed through the arteries each time the heart beats.

How To Measure Blood Pressure

1. Wrap the blood pressure cuff around the patient’s upper arm, ensuring that the appropriately marked side is down.

2. Check to ensure that the proper size of blood pressure cuff is being used. This will also be marked on the blood pressure cuff, as shown below.

The effect of cuff size on blood pressure measurement in adults.

Sprafka et al. (1991) measured blood pressure differences by cuff size in 181 adults aged 25 to 74 years, who were allocated to a random sequence that involved the measurement of blood pressure using a small cuff, a large cuff, and an appropriate cuff as determined by standardized arm circumference measurement. Systolic and diastolic blood pressure were underestimated by 3-5 mm Hg in men and 1-3 mm Hg in women when the cuff was one size larger than appropriate. Systolic and diastolic blood pressure were overestimated by 2-6 mm Hg in men and 3-4 mm Hg in women when the cuff was one size smaller than appropriate. In addition, 30-40% of subjects were “misclassified” when blood pressure cutpoints were used to define hypertension.

TL;DR – If the cuff is too big, the pressure will be incorrectly low. If the cuff is too small, the pressure will be incorrectly high.

3. Ensure that the patient’s blood pressure is measured at the same level as the heart.

The effect of arm position on blood pressure measurement in adults.

Netea et al. (2003) measured blood pressure differences when the arm was held at different heights. A progressive increase in the pressure of about 5 to 6 mm Hg was found as the arm was moved down from the horizontal to downright position. These changes are exactly what would be expected from the changes of hydrostatic pressure (the ‘weight’ of blood pooling above the measurement point).

TL;DR – If the arm is too high, the pressure will be incorrectly low. If the arm is too low, the pressure will be incorrectly high.

4. Close the screw valve on the blood pressure cuff (“righty-tighty”) and begin to inflate the cuff by squeezing the bulb repeatedly. Place the diaphragm of your stethoscope over the brachial artery on the inner part of the arm (see diagram). It is important to fully extend the patient’s arm in order to bring the artery close to the surface of the skin. You will begin to hear a heart beat through your stethoscope. Continue to inflate the cuff until the heart beat disappears.

Why do you only hear heart sounds with your stethoscope while inflating the blood pressure cuff?

When fluid moves through a channel, turbulence is created. When the flow is laminar (straight), this turbulence is minimized, as all of the flow is in the same direction. When the shape of the channel is irregular (such as when an artery is constricted at one point by an inflated cuff), the flow of the fluid becomes less laminar, and more turbulent. This same phenomenon can be seen when watching the flow of a river. When the river is wide and straight, the river appears to run smoothly. When the same river becomes more narrow or tortuous, the speed increases and there is an increased amount of turbulence on the water’s surface.

This turbulence makes noise (think of a turbulent river versus a lazily moving river). The turbulent noises (Korotkoff sounds) are what you are hearing with your stethoscope.

5. While watching the pressure gauge on the blood pressure cuff, loosen the screw valve (“lefty-loosey”) and slowly relax it until you begin to hear the heart sounds again. This is the systolic pressure.

6. Continue to relax the cuff until the heart sounds disappear. This is the diastolic pressure.

Report the blood pressure as systolic over diastolic.

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About Michael Spigner MD

Michael Spigner is a resident physician in the Emergency Department at the University of Cincinnati (USA). He currently serves multiple prehospital roles, including Team Physician for Cincinnati Police Department SWAT, Flight Physician for UC AirCare, and Assistant Medical Director for Reading Fire Department and Evandale Fire Department. He is a former Ambulance Unit President of the Manhasset-Lakeville Fire Department (NY), where he served as a firefighter, EMT-Critical Care, and member of the Technical Rescue Team for seven years. He has also held national leadership positions with the Emergency Medicine Residents’ Association, including as the Chair of Prehospital and Disaster Medicine. He is a former Assistant Medical Director of the Colerain Township Fire Department (OH). He has a vested interest in promoting evidence-based medicine in the prehospital environment, and empowering prehospital providers to advance their field through critical thinking and personal accountability.